1. Field of the Invention
The invention relates to a high adhesive strength acrylate-based coating that contains a small quantity of a polycarbonate-based TPU. More particularly, the coating is applied to produce an intermediate imbibable bifocal surface that is subsequently contacted with a photochromic solution.
2. Description of the Related Art
In-situ coating via direct injection, so called in-mold coating, has been successfully used in other industries such as automotive, construction, office products and outdoor equipment industries. It was originally developed to improve the surface appearance of sheet molding compound (SMC) parts molded by compression molding. In more recent years it's been applied to injection molded thermoplastic parts.
U.S. Patent Application Publication 2003/0227109 refers to a process for compressive decoration molding applied to the exterior of an injection molded box. The resulting decorative film is a coating along the top and sides of the box, that is, on surfaces perpendicular to the mold parting line. The high viscosity opaque coating is compressed at a controlled rate to provide uniform thickness on all part surfaces. The publication is directed to solving a problem of specific product configuration and does not relate to forming ophthalmic lenses which require optical clarity and varying thickness.
U.S. Patent Application Publication 2006/0151911 describes an injection molded part that is coated with a lacquer layer. The lacquer is injected under pressure into a closed mold that is treated with a mold release agent. Solvent free lacquers with short shelf lives are prescribed through implementation of a reaction injection molding (RIM) process. The injected lacquer is cured at an elevated pressure, whereby the coating expands and then shrinks, making it difficult to control final coating thickness.
U.S. Patent Application Publication 2005/0089630 describes a lens made by a first process which is then gasketed to receive a surface cast layer. The original lens must be cleaned and dried in order to apply a tie coating before gasketing. The tie coating is also subject to a pre-cure to partially polymerize the coating to improve its ability to tie the original lens to the cast layer. The described compositions and methods suffer from complexity and too many steps.
U.S. Patent Application Publication 2006/0118999 describes a simultaneous molding/overmolding technique utilizing a reversibly engageable rotatable core having identical molds on multiple sides. The technique is designed to allow the molding process to occur on one core side at the same time as the overmolding process takes place on another core side. The parallel molds are not well suited for producing articles of optical quality because the hardware configuration does not allow for interchangeable mold inserts that are needed to produce lenses having different base curves. In addition, since one clamp is used to close both parallel molds, there is only a single clamp force profile which represents a compromise between the two distinct types of molding operations.
U.S. Pat. No. 7,077,985 describes a film insert molding process for adding photochromic or polarizing functionality to an injection molded lens. The functional film is sandwiched between two protective polymeric layers and pre-shaped before being placed into the bifocal injection mold cavity. The preparation of the film involves multiple steps and requires additional resources to keep the film inventory clean prior to use. U.S. Pat. No. 6,367,930 describes a separately prepared photochromic TPU film and a polycarbonate ply that are collectively inserted into a mold for a film insert process. In an alternate embodiment, a photochromic TPU that is injected, for example via two injection molding machines or a two-shot injection molding machine. In these processes, an intermediate imbibable bifocal lens is not prepared.
This closed mold method of in-mold coating has an advantage, in that the coating is able to cure at the same time as the part is cooling. Since access to the part is limited by the closed mold, all of these systems introduce the coating at the top of the mold cavity with the coating injector being located near the parting line. The preferred molding machine configurations having a vertical parting line, with the movable mold half being reciprocated in a horizontal direction. An example of such machine configuration can be readily seen in U.S. Pat. No. 6,180,043. This patent is concerned with high gloss, opaque coatings, containing as much as 30% and up to 45% titanium dioxide and other pigments. Clearly, for such coatings in which one sees only a highly reflective outer surface, there is no requirement for uniformity or transparency, as with an optical coating. In addition, mold cleanliness and contamination are not issues due to the higher viscosity of pigmented coatings. U.S. Pat. No. 6,180,043 utilizes multistage declining clamping force during the coating cure stage, while WO 2003/031138 utilizes multistage declining clamping force during the plastic injection stage.
The remaining closed mold patents describe various molding machine modifications to contain the coating within the mold cavity. U.S. Pat. No. 6,676,877 provides a coating containment shroud along the runner passageway to prevent coating solution from contaminating the liquid resin in the screw. The International Publication WO 2004/048068 relates to retrofitting existing molds for use with an in-mold coating system. Published U.S. Patent Application 2003/0077425, Published U.S. Patent Application 2003/0082344 (corresponding to International Publication WO 2003/035354), and Published U.S. Patent Application 2003/0099809 all relate to the addition of a rim feature on the molded item, to seal the parting line and prevent coating solution leakage. While these solutions are adequate for center-gated molds and automobile parts, they are unsuitable for the use in edge-gated molds or the product configuration of optical lenses.
U.S. Pat. No. 5,943,957 discloses a method for pad printing inked images onto injection-molded pieces while they are still in the mold. The patented method relates to conventional ink that air dries, and does not involve an optical grade coating that will be spread over the lens by re-clamping the mold inserts and allowing the coating to cure via the retained heat in the mold block. Published U.S. Patent Application 2003/0152693 discloses pad printing of lenses, but applies a UV or microwave curable coating on cast lenses which are totally divorced from any mold contact.
The present invention describes equipment and provides methods and formulation to apply a coating or thicker layer on the surface of an ophthalmic lens while it is still in the mold. The coating is thermally cured by the heat from the mold and the residual heat from the thermoplastic lens to provide an imbibable bifocal surface. In order to address adhesion and performance defects, the formulations of the invention include fractional amounts of a TPU in an acrylate-based thermoset formulation.